This book is concerned with what has been called the "Monitor Theory" of adult second language acquisition. Monitor Theory hypothesizes that adults have two independent systems for developing ability in second languages, subconscious language acquisition and conscious language learning, and that these systems are interrelated in a definite way: subconscious acquisition appears to be far more important. The introduction is devoted to a brief statement of the theory and its implications for different aspects of second language acquisitions theory and practice. We define acquisition and learning, and present the Monitor Model for adult second language performance. Following this, brief summaries of research results in various areas of second language acquisition serve as both an overview of Monitor Theory research over the last few years and as introduction to the essays that follow.